1. Field of the Invention
The present invention relates to a method of manufacturing a rigid polyurethane foam molding for energy absorption and a mold used in this method, and more particularly to a method of manufacturing a rigid polyurethane foam molding suitable for an energy absorbent pad to be mounted inside a door trim of an automobile to absorb energy upon collision of the automobile and thereby protect a passenger, and also to a mold used in this method.
2. Prior Art
Conventionally, expanded thermoplastic resin products formed from thermoplastic resins such as polystyrene, polyethylene, and polypropylene are used to absorb impact energy upon collision of the automobile. However, such an expanded thermoplastic resin has a problem such that its energy absorbing characteristics vary widely according to its surroundings and the temperature upon using, and the amount of energy absorption is remarkably decreased especially under high-temperature conditions. Furthermore, the relation between strain and stress representing the energy absorbing characteristics in the expanded thermoplastic resin is such that stress tends to increase continuously with an increase in strain, so that the expanded thermoplastic resin is unsuitable for use in the situation that a large amount of energy absorption is desired in the stress range up to the upper limit.
On the contrary, a polyurethane foam as a thermosetting resin, especially, a rigid polyurethane foam having a high cross-linking density has many advantages such that the range of variation in energy absorbing characteristics with temperature is small, a so-called plateau value is large, and the polyurethane foam is suitable for use under high-temperature conditions.
The relation between strain and stress in the conventional rigid polyurethane foam is better than that in the expanded thermoplastic resin in such a manner that stress is less changed with a change in strain. However, the conventional rigid polyurethane foam is not yet satisfactory in its energy absorbing characteristics. For example, the characteristics include a yield point through which stress once increases and thereafter decreases with an increase in strain. It is therefore desired to develop a rigid polyurethane foam having such energy absorbing characteristics that stress is constant as possible irrespective of a change in strain and the energy absorption is high.
The present assignee has already proposed a rigid polyurethane foam having no yield point in the relation between strain and stress, so as to satisfy the above desire (Japanese Patent Application Kokai No. 5-331365, for example).
The rigid polyurethane foam thus proposed is usually manufactured as a slab stock foam, and this slab stock foam is cut into a desired shape suitable for an energy absorbent pad. However, the slab stock foam cutting process requires a considerable amount of man-hours, and also generates foam particles due to cutting, which particles cause a serious problem in disposal.
Furthermore, since the slab stock foam is cut with a cutter, there is a limitation in shape of the product, so that it is difficult to obtain a complex shape. In addition, the foam particles generated in cutting the slab stock foam adhere to the resulting product, and they sometimes scatter or adhere to the surface of another member such as a door trim, for example, in assembling the product and the door trim or in any afterprocessing, thus causing bad appearance to the door trim, etc.